Short bursts of electric power are conventionally supplied by means of capacitors. Capacitors are suitable for applications such as photo-flash units which involve relatively small amounts of stored energy, ranging from several joules to several kilojoules. However, capacitors are too large and expensive for applications involving several megajoules of stored energy. Such applications require energy supplies in which the energy is derived from a storage inductor. Megajoule pulsed power applications include electromagnetic launchers, such as aircraft catapults and guns, percussion welding machines, pulsed magnetic field metal forming machines, pulsed lasers, and x-ray and radar systems. Since storage inductors dissipate their energy in a matter of seconds, they must be energized much more quickly than capacitors; this requires rotary generators capable of delivering short bursts of high power. Conventional generators cannot be forced to operate above their rated output, even for short bursts, because power output is limited by magnetic saturation of their iron circuits. In addition, only a small fraction of available volume is used for conductors.
Two types of machines exist today which are capable of high power pulse duty: homopolar generators, also called asynchronous generators, or Faraday disk generators, which are dc generators in which a conducting disk or cylinder rotates in a stationary magnetic field and compulsators. Homopolar generators operate at very low voltage and very high current, and require contact brushes for their output current. They have the additional disadvantage of slow response to excitation control, and inability to use switching elements for output current control. Their usefulness is therefore restricted to loads having very low impedance. Compensated pulsed alternators, also called compulsators, can operate at high voltage, but produce very short pulses of power which last only a fraction of one revolution, typically milliseconds. Existing generators, both dc and ac, are designed for maximum efficiency and continuous operation, and incorporate an iron magnetic circuit to minimize the excitation current, and only a very small fraction of the available rotor and stator volume is occupied by conductors, typically about one percent. Generators which do not use iron are certain types of superconducting generators. Since superconductors have no ohmic losses the excitation field can be produced easily without the use of iron.